1. Field of the Invention
The present invention relates, in general, to the lower end plug for an annular nuclear fuel rod and, more particularly, to a lower end plug, which plugs up the lower portion of an annular nuclear fuel rod so as to prevent the fusion gas filled between the cladding tubes of the annular nuclear fuel rod from leaking out.
2. Description of the Related Art
A nuclear fuel assembly is composed of a plurality of nuclear fuel rods. The conventional fuel rod has cylindrical type, in each of which a uranium sintered pellet (or a cylindrical fuel pellet) is inserted.
A new concept of fuel rod is annular type which is internally and externally cooled nuclear fuel rods.
The pellet of each annular nuclear fuel rod has a low peak temperature due to a thinner pellet thickness and a wider heat transfer area, and thus a relatively higher safety margin, as compared to that of each cylindrical nuclear fuel rod.
FIG. 1 is a schematic front view illustrating a conventional cylindrical nuclear fuel assembly. FIG. 2 is a schematic top plan view illustrating the configuration in which nuclear fuel rods of the cylindrical nuclear fuel assembly of FIG. 1 are supported by spacer grids. FIG. 3 is a schematic top plan view illustrating a bottom end piece of the cylindrical nuclear fuel assembly of FIG. 1.
Referring to FIG. 1, the nuclear fuel assembly 10 includes nuclear fuel rods 15, spacer grids 30, guide thimbles 40, a top end piece 50 and a bottom end piece 60.
Each nuclear fuel rod 15 has a structure in which an uranium sintered pellet or a fuel pellet (not shown) generating high heat through nuclear fission is enclosed by a zirconium alloy cladding tube (not shown).
Each nuclear fuel rod 15 has lower and upper end plugs 71 and 72 which plug up the lower and upper portions thereof so as to prevent the fusion gas filled between the cladding tubes from leaking out.
Referring to FIG. 2, each spacer grid 30 has a plurality of rectangular grid straps 31, each of which has a spring 32 and dimples 33. Each nuclear fuel rod 15 is inserted into each spacer grid 30, and is contacted and supported by the spring 32 and dimples 33 of each grid strap 31.
Each dimple 33 has a strong elasticity, while the spring has a weaker elasticity than a dimple 33. The strong elasticity of the dimple 33 keeps the nuclear fuel rod 15 positioned in a transverse direction, and the weak elasticity of the spring 32 provides a supporting force to the nuclear fuel rod 15.
The guide thimbles 40 are arranged in the nuclear fuel assembly, and provide moving passages for the control rods (not shown).
The top end piece 50 and the bottom end piece 60 fix and support the nuclear fuel assembly 10 to and on the upper and lower structures of a core (not shown).
Referring to FIG. 3, the bottom end piece 60 has the shape of a porous plate in which a plurality of flow holes 61 is regularly formed. The coolant is supplied to the spaces between the nuclear fuel rods 15 through the flow holes 61.
FIG. 4 is a schematic cross-sectional view illustrating an annular nuclear fuel rod. FIG. 5 is a schematic perspective view illustrating a lower end plug that can be applied to an annular nuclear fuel rod. FIG. 6 is a schematic cross-sectional view illustrating the arrangement of the annular nuclear fuel rods of FIG. 4.
Referring to FIG. 4, the annular nuclear fuel rod 20 includes an annular fuel pellet 21 generating high-temperature heat by means of nuclear fission, an inner cladding tube 22 coupled to an inner circumference of the fuel pellet 21, and an outer cladding tube 23 coupled to an outer circumference of the fuel pellet 21.
An inner space of the fuel pellet 21 acts as an inner channel 24 through which the coolant for cooling the high-temperature fuel pellet 21 flows.
The annular nuclear fuel rod 20 is coupled with upper and lower end plugs 70 and 80 at the upper and lower ends thereof, so that the fusion gas filled between the cladding tubes 22 and 23 of the annular nuclear fuel rod 20 is prevented from leaking out.
Referring to FIG. 5, the lower end plug 80 has the shape of a hollow cylinder or a hollow pipe in which a space 81 (hereinafter, referred to as “main hole”) is formed. The lower end plug 80 is provided with a plurality of inflow holes 82 in an upper wall thereof, and a plurality of fastening holes 83 in a lower wall thereof. A filter pin 84 is inserted into each fastening hole 83 in a criss-cross shape so as to prevent debris from flowing into the main hole 81.
Referring to FIG. 6, spaces between the guide thimbles 40 and the annular nuclear fuel rods 20 are sub-channels 11. The coolant for cooling the high-temperature heat generated from the annular nuclear fuel rods 20 flows in a transverse direction and in a longitudinal direction, too.
The transverse direction refers to a direction perpendicular to an axis of each annular nuclear fuel rod 20.
The transverse flow of the coolant is determined by the ratio (G/P) of a gap G to a pitch P between the annular nuclear fuel rods 20. When the ratio is great, the transverse flow resistance of the coolant is small. In contrast, when the ratio is small, the transverse flow resistance of the coolant is great.
Hereinafter, only the nuclear fuel assembly to which the annular nuclear fuel rods 20 are applied will be described.
The nuclear fuel assembly 10 having the aforementioned structure generally burns for three cycles in the core of the nuclear reactor. Each spring 32 of the spacer grid 30 is subjected to a change in its mechanical properties by radiation exposure with a lapse of two cycles, so that it loses its elastic force.
When not supported by the spring 32, each annular nuclear fuel rod 20 falls down due to its weight until it settles on the bottom end piece 60, which is called a “bottoming” phenomenon.
When this “bottoming” phenomenon occurs, the main hole 81 of the lower end plug 80 can be blocked by the bottom end piece 60. Thus, the coolant may not flow into the main hole 81 of the lower end plug 80, but only into the inflow holes 82 formed in the upper wall of the lower end plug 80.
Since the inflow holes 82 are formed in the upper wall of the lower end plug 80 which is perpendicular to the direction in which the coolant flows, high flow resistance is occurred. For this reason, the flow rate of the coolant that flows into the inner space, i.e. the inner channel 24 of each annular nuclear fuel rod 20 may be reduced, so that the cooling capability in the inner channel can be reduced.
Further, the nuclear fuel assembly 10 composed by the annular nuclear fuel rods 20 has a narrower gap and a greater outer diameter, as compared to that composed by the cylindrical nuclear fuel rods 15. As such, the transverse flow resistance is increased relatively, so that it is difficult for the coolant to flow between the neighboring sub-channels in a transverse direction.